Process fob the dehydrogenation



Nov.- 7, 1944.

W. A. SCHULZE ETAL 'PROCESS' FOR THE DEHYDROGENATION 0F HYDROCARBONSFiled Sept. 25,' 1940 Patented Nov. 7, 1944 UNiTE'D STATES PATENT OFFICEPROCESS FOR THE DEHYDROGENATION F HYDROCARBONS Walter A. Schulze and`lohn C. Hillyer, Bartlesville,- Okla., assignors to Phillips PetroleumCompany, a corporation of Delaware V' Application September 23, 1940,Serial-No. 358,008

(c1. 26o-eso) 2 Claims.

This inventionrelates to the production of the valuable dioleiinichydrocarbon butadiene from suitable four-carbon-atom hydrocarbons. Itrelates more particularly to the process of 'dehydrogenating normalbutane in successive 'stages to produce successively butenes andbutadiene. f In 1a more specific sense this invention is concerned witha new and improved process in which the mono-clelin butene-2 obtained asone of the products from the catalytic dehydrogenation of normal butaneor;of the C4 fraction of thermally cracked gases is separated byfractional distillation and is subsequently dehydrogenated in al secondcatalytic step to produce a good yield of butadiene. It has already beenproposed butadiene by the vcatalytic treatment of the total butenemixture resulting from the dehydrogenation of normal butane. However, ithas been the practice of those attempting to carry lout asV a process'to first dehydrogenate butane to produce mixed butenes and to recyclethe treated va'pors until the desired concentration of butenes wasreached o r the desired percentage of the butane charge had beenconverted. Because of the limitations -imposedbythermodynamic,equilibrium, it has Anot been possible to secure,satisfactorily pure mixed butenes by this Iprocedure without resortingto such severe conditions and excessive recycling* thatconsiderable.quantities of the butane and butenes were decomposed, and thereresulting yields of butenes were not commercially feasible. j

v Also attempts have been made to produce mixed butenes bydehydrogenating4 normal butane and separating the resulting butenesafter each passage over the catalyst for further dehydrogena- `tion.Such attempts have involved various compleX solvent extraction andchemical separation methods which have proved expensive and generallyunsatisfactory.

We have now discovered a novel process for the production of butadienewhich eliminates the unsatisfactory practices of the art to date. Ourinvention not only provides for the economical production of butenes bycatalytic dehydrogenation but also includes the segregation of butene-2as charge to aseconddehydrogenation treatment [to produce butadiene. Bythe `practice of our infractionation, thus increasing the per passrecovery of substantially pure butene-2 by fractional distillationlwhile butene-1' Aand n-butane together with a portion of the butene-2are recycled tothe dehydrogenation catalyst. It is therefore an objectof this invention to convert normal butane to butadiene in two stages ofdehydrogenation using a stream of substantially pure butene-2 separatedafter isomerization of the products of the iirst dehydrogenation step asa charge to the second dehydrogenation step.

In a mixture of C4 hydrocarbons such as is produced 'by dehydrogenating4normal butane the boiling points of the components are as follows:(from- Physical Constants of Hydrocarbons, volurne I-EgloiT-ReinholdPublishing Company, 1939)'.

0 F. Bute'ne-1 2'0 Butadiene 23 n-Butane 31 Trans-butene-2 33Cis-butene-2 39 It. has already been proposed in copending applicationSerial No. 352,787 led August 15, 1940, to separate butene-1 from thislmixture by fractional distillation. However, the equilibriumconcentrations established at dehydrogenation temperatures are such thatof the total butenes produced, the predominant percentage is the 2-isomer. The following tabulation shows equili-brium concentrations of.butene-1, transbutene- 2, and cis butene-2 over a wide range ortemperatures.

Concentrations in mol per cent of total butenes VTemperature, FF.' y YTrans bu- Cs bu- Butene'l tene-2 tene-2 while the dehydrogenation ofnormal butane in the temperature rangefoflQOO to 1200" F. -yields highconcentrations of. butene-1 this represents only aboutvone-third of thetotal butenes, the rel maining two-thirds being cis and trans isomers`of butene-2.

butane can be adjusted by isomerization to in- It may be seen also, inlthe above tabulation that at low temperatures the-equilibriumconcentration ranges up to more than 97 percent of butane-2. We have nowdiscovered that the rapid attainment of the low-temperature equilibriumconcentrations of butene-2 is possible by passage of the partiallycooled eluents from the first catalytic dehydrogenation step throughactive isomerization catalysts at temperatures in the range of 250-500F. Even lower temperatures may sometimes-beused in this isomerzation.step. We have found it possible to convert nearly' all of the butene-lpresent in a C4 hydrocarbon mixture of the type described above into thehigherboiling butene-2 isomers, thereby .concentrating the butenes onthe higher-boiling side offnor'xnal butane.

We have found that after-the applicatignof this step of catalyticisomerization at the proper temperature levels to the partially cooledvapors from the dehydrogenation oflnormalgbutanefwe are able to secure asatisfactory separationhbetween normal butane and the butene-Z isomers.by :means of ordinary .,f-:roetioual distillation- .AlthoughCleanseParetion .between-@nonna butanefiand trans floutene-.Z f een beAobtained only wwith;columns V:employing somewhat suore than A10oplates-:wade notfnormellyottepipt.toolean ,separationbut conduct thefractionation -insuoh manner .1 that :the bottoms product `is -oomposedLpredoniinately-,of tutelle-2. By our-.iprocess,l the .overhead fractioncomprising uneonverted vhu- Atene-,1. normal butane :and a. little transbutene-.z is: reoyeledftorthe; initial dehydrogenation gases instead ofn-butane in our process the steps are not materially altered. In such afraction comprising n-butane and butenes, the butene concentration isnot usually great enough to Justify the segregation of butene-Z prior todehydrogenation, and the entire stock may be dehydrogenated according tothe first step of the process. Obviously, if such a C4 fraction containsnormal `butenes in concentrations equal to or `greater A than thoseproduced by the rst dehydrogenation voperation, the raw feed may becharged directlyl tothe, vsecond step of our process. Any isobutenesterazvvhile. tl;1eY bottomsfffreotion kco.inprising-fithe '-buteneez`rich `fraction isxthe ,oharsezfor .the .ond dehysirogenation step, toproduce rbutadlV ne.

Operating-costano extremely ,lowfdue to (1), the 1,-.'

catalytic isomeriaationffstep fis performed .ont #the effluents from thedehydrogenation step `during ,the normal cooling operation in suchmanner that no.additional heat .inputis requiredaand '12)l .thefractionation step has been greatlysimf '.pliiied .by .concentrating.the ,butenes `on the highereboiling side. of l-norrnal butane.

Inour. processy .the recycled butane-1` doesnot build upi..i..n.thesystem. becausetheamount recycled .-is Ifor `below .the .equilibriumconcentration at -dehydroeenation te.noperature..s4 and .furtherconversion of, ,.nonnai butane reesteblishes .the equilibrium.

. 11n .its broader.. aspects, our. invention comprises oonductinathe.conversion of n-butane .to butadienebyaserlesof ysteps llstedbelowinthe order employed: 1) catalytic. dehydrogenation ofl `nhutane; A` 2).catalytic isomerization. .ofl .theehuents from (1) to increase theconcentration 0f butene-2 therein; (3) separation of the efllllent` lfrom l(2) byfractional distillation into an overhead fraction`v,comprising butene-l, trans butene.2. and nebutane andabottornsfractioncomprising cis and trans butene-Z with some n-butane;-(4) continuously recycling the overhead of (5) after separation ofbutadiene to the second dehydrogenation step'. In `the practiceot thisinventionany butadiene formed 4in stepy 1) win distill overheadalongwith the butane-1 andvk nbutane' fraction. If the lulldlltity'thus'.formed ris. large enough to maken recovery step; Afs.ius..i.b le,

the butadiene may be absorbed-'fromthe recycle materalprior tore-introduction injaojthe iirstl dehydrogenation step.

-- -rf we use theCl' fraction from retlnery cracked present in thestocks mentioned above may be removed or utilized as desired prior totreatment by ourlprOceSs.

y'In order 4that the invention may be more clearly understood, referencewill be made to the figure 'which is ai flow diagram of the steps of theprocess.

e In the gure the raw n-butane or suitable Cl -fhydrocarbon feedcomprising butane and butenes entersyby line I into. heater 2 where thefeed is raisedtothe desiredI temperature. The hot va- --pors thenpass byline 3ginto catalyst cases `4 `containing a suitable dehydrogenationcatalyst.

Eromill the treated vapors pass throughl-ine 5 to cooler `6 where thevapor temperature is low.-

,through polymerseparator 8 wherein any heavy -material is .removedthrough line 9. The vapors .Ehen pass vthrough-line I0 into catalystchambers II containing .an isomerization catalyst. The

V,efuer1-ts :from I Ipass throughline I2 and in turn .through cooler I3,compressor I4, cooler I5 and into accumulator I6. From I6 the.condensate passes .to .fraetionatoror depropanizer l'lafor .the

Sopliitililgf Propoli@ und. ,lghifdimaterial 0V??- head .whilevthebottoms .comprisingy C3 and.C4 -rmaterialpasses ,through line I8 tovfractionator 1,8. .lh `flactionator laan overhead fraction pre-Jlduminantly lower boiling than trans butene-Z `and comprising butane-1,n-butane and some .trans 4buterjie- 2 istaken overhead whilel cis andtrans buttano-2 .and some n-butane constitute thebolztoms .fractionpredominantly higher boilring thantransbutene-Z. The overhead fractionis recycled `to the dehydrogenation step through linel. ,If`butadienererxioval is desirable the recyolematerial `passes .throughline, 22 into labsorption unit,A 2.3, and by line 24 `back into therecycle line 2D. ,Refrigeration .(not shown) may .be required- .'.I'hebutadiene addition product. is

removed by line 25. The conventional auxiliary 4.eauipnierit....foriracticnators l1 yand ls including heat exchangers, oondensers, reuX-accumulator and thelike is `familiar-to the art and Vthus ispot yshowunthis fiowldiagram.

-.The ,butenef2 concentrate.. .namely the bottoms .fraoton from i9..posses through line Z6 and together with `a diluent which may be .addedthrough line 21 enters heaterI .ZB-Wherethe-A stream .is ,heated.todehydroaenation temperatures. The hot vapors ,then passthrough line.,29 into ,catalyst ,ehambersll .contaminare dehydrogenotion-.oato-.1i/st. .The dehydroeenated, .vapors leave Vthrouah line .3l.andtheanass in .turn .through cooler 3,2,

.compressor 3.3.. .and .Cooler 34 .into accumulator ,35 .Where propaneand. heavier material f. are l condensed and..lightermaterialis vented-.Theliduid from 35. may pass `thrcuieh line- 36 into .fractionator orAdeproponizer 31. .In 3l.propane,=.and

liehtermaterlal is ,taken oyerheafsl.and-uoeyv enter line 2,6 throughline 38 and serve..as..d.i,luent.aas or alternately.. may be ventedthrough. line 33.

TheCi fraction'` from .the bottom :of fractionator The Ciiresiduum fromthe butadiene lAlternately, the Ca-C4 liquidffrom accumulator '35 may'pass with` suitable refrigeration (not shown) directly to the butadieneabsorption unit 4| `through lines 44 and' 4I).y In this case, butadieneis absorbed from the totalcondensate without fractionation andthe C3-C4residuum passes as` recycle to thejcatalyst .through line t2.` Thevpropane thus included v.may serve` as diluent in .the dehydrogenationto I'replace all or a part of ithediluent indicated as added throughlines 21 :and 38.. f x

,In the1 `operation of the first dehydrogenation step, the hydrocarbonvapors may be subjected `to two or more successive treatments ,withdehydrogenation catalyst in'a series of catalyst chambers,

or the vapors or any portion thereof may be recycled with thefresh feedr vapors through-the. .catalyst chambers. be applied to the recycled:vapors ifdesired.

Some additional heat may 'Other possible arrangements of the convention-Y. al equipment used in the practice of our invention willbe apparent tothose skilled in the art, and thusare held within; the scope of ourinvention;

Also, the, conditions of temperature, pressure, flow rate and the ',likeused in operating this requipment will, depend largely` on the selectionofccatalyst' to bewusedin each step`and on the desired degree ofconversion, since each catalyst particularly difficult reducible oxides,but includingoxides,A rof metals in groups II toVIII inclus-5n ive ofthe periodic system, activated or lustrous carbon, clays. somegsilicatesand many others. The great vvarietyjof oxide catalysts makes them of themost importance.

For the isomerization step, catalysts active in relatively lowtemperature ranges are preferred. These include catalysts of an acidicnature; either mineral materials containing acidic substances such ascertain clays and natural silicates, acidic salts such as aluminumphosphate and the like or strong mineral acid catalysts are useful.Other types of catalysts may be used under substantially anhydrousconditions, including alkali and alkaline earth metal oxides, but theacidic catalysts are generally more uniform and more active.

In the practice of our invention the charging stock to the initialdehydrogenation operation is usually heated to temperatures in the rangeof 850 to 1200" F. and passed over the catalyst at such velocities thatcontact time is quite short of the order of 0.5 to seconds. Lowsuperatmospheric pressures of about 5 to about 50 pounds gage arenormally used, although higher pressures up to 200 or 300 pounds gagemay be used if desired. Conditions of operation are selected withreference to economic and technical factors in any given installation.

In the isomerization step of our process the y eilluent vapors from theinitial dehydrogenation are cooled and. any heavy polymerformed maybeseparated prior to passage in vapor phase over theisomerizationcatalyst at temperatures within the range of 200 to 600 F.The temperature is maintained at the lowest level which will give rapidisomerization and a minimum of polymerization reactions since theequilibrium favors vbutene-2 at lower temperatures. Low superatmosphericpressures of 5 to 50 pounds gage are suitable in theisomerizationreaction, and flow rates equivalent to 0.5 to 51'liquidvolumes of charge :per hour volume of catalyst are usually satisfactory.The eflluents from .the isomerization step are further cooled,compressed-and partially condensed prior` to the vfractionation forseparation `of butene-2 for the second dehydrogenation operation.

In the second dehydrogenation step, the charge stock is heatedsufficiently to maintain temperatures between about 1050 and 1350 F. inthe catalyst chambers. The catalysts used may be those which give asuitable degree-of conversion of butene-2 to butadiene and which do notinduce excessive polymerization and cracking reactions. Further, it isusually desirable to maintain low partial pressure of butene-2 in thecharg to the second dehydrogenation step, for example by addition of aninert diluent, in order to suppress deleterious side reactions involvingthe unsaturated hydrocarbons.

Ordinarily two or more catalyst4 chambers would be provided for eachcatalytic step. Those chambers not in service would be under preparationfor subsequent use, either by replacement or spent catalyst or byregeneration. Regeneration is contemplated for the dehydrogenationcatalysts whenever the activity has declinedV t any predetermined level.i

The regeneration may be carried out by means such as controlledtreatment with an oxygencontaining gas. l 1

'I'he following example will serve to further illustrate one method ofpracticing` our invention.

Example Normal butanewascharged to the system. diagrammed in thedrawing, yoperated. ata pressure of 30 pounds per square inch gauge. Theheated vapors emerged from the furnace and entered the catalyst cases at1120 F. The multiple cases were filled with calcined 6-14r mesh bauxiteto such depth that the total pressure drop was of the order of fivepounds or less, and the temperature of the entire bed maintained withinthe range of about HOO-1120 F. The butane was processed at a spacevelocity of 1000 volumes (STP.) per hour, per volume of catalyst,equivalent to a contact time of approximately 1.7 seconds. About 15 percent of the butane charged was converted to butenes, and the run wascontinued for 24 hours before catalyst activity had declined to a pointat which regeneration was necessary.

'I'he efiluents were cooled quickly to about 450 F. and passed over analuminum phosphate catalyst to isomerize butene-1 to butene-2. 'I'hevaporsemerging from this tower had approximately the equilibriumconcentration, or about 14 per cent of the butene content as butene-1and 86 per cent as butene-2. l

The eiiluents from this tower were depropanized and submitted tofractionation in the highly eflicient tower diagrammed. The cut was madein the trans butene-2 fraction, separating as `bottoms fcis butene"2 andparty of the :trans `buten'eeZ lto- `ethenwith some .butane The lightgases were separated-from the butane-butene-l overhead `Lanclthelatterwas recycled. When the recycle fzstreamf ofbutane 1 andwbutene-l hadbeen adm'itted and a steadystate of vdehydrogenationisomerization hadbeenestablished, hydrogen. and `light Lfgases separated accounted for fabout rthree percent lay-weight of the-charge.' Butene-Zsepavratedfromthezbottomof the tower accounted `rfor f.21per cent oi thev totalvcharge tothe heater'with 4which additionalbuteneequivalent to 2. percent `of the 'charge-was associated. The butene-l and transf'butene-2`recycled` amounted to 24 per cent of the charge,y whilevthevbutane'recycled was 50 per cent and fresh added butane was 26percent of the charge. Thus an ultimate yield of about 81 per cent of thebutane charged to the first dehy- "drogenation was' obtained as butene`2in 5 the charge to the Vsecond dehydrogenation,

' The butene-Z-butane fraction was then charged vto the seconddehydrogenaton step shown in the drawing, with sufcient volume ofsubstantially inert, diluent'gas to yreduce `the partial pressureof-butenes to 25 per cent of the total pressure. vThis stock wasvprocessedat a pressure of 5 pounds `gage and at a temperatureof1125-1130" F. over A bauxite catalyst. A space velocity of 1400volumes per hour was employed. About 28 per cent conversion per pass ofthe butene was obtained of which"'50 per -cent was butadiene and thevremainder light gases, polymer and coke. The C4 {fi-action `afterremoval of fixed gases and the :ex-

ltractionjo'fthe butadiene was recycled; A cycle l-oi sixylioursfoperation followed by regeneration .wagused in' 'this stage;`the streambeing changed "to-a; fresh set of catalyst gases at the endof'this period and the first catalyst regenerated by conytrolledcombustion.

"The 'foregoing specification and example have disclosed and illustratedthe invention,` but since it is oiV generallywide application and thenum- .ber` o'f examples of results obtained by its-use with kavdehydrogenating- :catalyst under suitable conditions to produce'amixturecomprisingbutadiene, butene-l, bateria-2, and butane, passingsaid mixture over an isomerization catalyst under conditions Asuitableto promote the conversion of butene-l to butene-Z,v fractionating theeilluent from the isomerization stepto produce an overhead fraction4predominantly lower-boiling lthan trans butene-Z and a bottoms fractioncomprising essentially cis and transbutene-Z., recycling saidoverhead-fraction after absorption of' butadiene `therefrom fto theinitial `dehydrogenationfstep alongwith fresh-'butane feed, treatingvsaid bottoms fraction in asecond dehydrogenation step to convert asubstantial proportion of the butene- 2 to butadiene, separating thelbutadiene -ilrom vthe products of the second dehydrogenation step priorto recycling the unconverted C4 material therefrom to the seconddehydrogenation 'step along with `fresh added butene-2 stock'and'nallycombining the butadiene sol recovered from' the two dehydrogenationsteps.

.2. A process "for preparing butadiene from hydrocarbon mixturescontaining major proportions of n-'butane which comprises catalyticallydehydrogenating` said mixture under suitable conditions to convert asubstantial portion'ofthenbutane to butenes-l and 2 with thesimultaneous vproduction of some butadiene, catalytically isotion-steptheunconverted butene stock along with freshadded butene-Z feed.

' WALTER A. SCHULZE.

JOHN C. HILLYER.

